Leave.EU and an insurance company owned by its founder Arron Banks have been fined £120,000 over data law breaches.
It represents a reduction in the £135,000 total previously announced by the Information Commissioner’s office.
The pro-Brexit Leave.EU group’s £60,000 fine was reduced to £45,000 after “considering the company’s representations”, the ICO said.
Leave.EU said it was a “politically motivated attack against our involvement in Brexit”.
A spokesman said it was “disappointed but not surprised” and would be appealing against the fine in court.
The fines follow an Information Commissioner investigation into the misuse of personal data by political campaigns.
The report says more than a million emails sent to Leave.EU subscribers contained marketing for the Eldon Insurance firm’s GoSkippy services. Eldon Insurance has been fined £60,000 for the breach.
In addition to the £45,000 fine for that breach, Leave.EU was also fined £15,000 for “using Eldon Insurance customers’ details unlawfully to send almost 300,000 political marketing messages”.
The ICO confirmed on Friday it would be reviewing how both are complying with data protection laws, including looking at how personal data is processed and staff training.
The watchdog will also be interviewing the directors, staff and data protection officers at both organisations.
Eldon Insurance has also been issued with an enforcement notice, ordering it to take steps to make sure it complies with electronic marketing regulations.
Information Commissioner Elizabeth Denham said: “It is deeply concerning that sensitive personal data gathered for political purposes was later used for insurance purposes; and vice versa. It should never have happened.
“We have been told both organisations have made improvements and learned from these events.
“But the ICO will now audit the organisations to determine how they are using customers’ personal information.”
In November, Mr Banks defended himself on Twitter. The Information Commissioner’s Office (ICO), he said, had found “we may have accidentally sent a newsletter to customers” but “no evidence of a grand data conspiracy”.
He added: “Gosh we communicated with our supporters and offered them a 10% Brexit discount after the vote! So what?”